The integration of a transmitter and receiver that communicate through a transmission line poses two major problems. The first problem has to do with the transmission line's throughput. It is known that for high frequencies, a transmission line determined by its characteristic impedance is used, commonly called Zc, and similar to the equivalent resistance. At its two terminals the line is connected to two amplifiers respectively called buffers. The input buffer amplifies the power of the signals to be transmitted and the output buffer rearranges the signals received and amplifies them to be correctly processed by the receiver. On the other hand, binary signals are transmitted according to various codes marked by leading and trailing edges that alternate according to variable durations in proportions that may be greater than ten. In the area of gigabaud, the transmission signals may vary within a frequency band in excess of 500 MHz. However, each duty cycle must be transmitted without garbling to ensure the integrity and fidelity of the transmission. This requires the throughput time for each buffer to be fixed, regardless of the transmission frequency in the required band, the wave shape and the quality of the leading and trailing edges.
The second problem has to do with the technology for manufacturing integrated circuits. Bipolar technology offers the advantage of having rather stable characteristics, but it consumes considerable amounts of power. Field effect transistor technology consumes less power, particularly that of complementary transistors, for example, CMOS technology, but it greatly dissipates characteristics among the same components of two different integrated circuits. Thus, in this case, the idea is to make the transmitter and the receiver operate in a manner that is practically insensitive to such dissipation. Moreover, it is desirable for the transmission device to be independent from the technology selected to manufacture the transmitter and receiver. For example, a receiver made using MOS technology should do an equally good job of receiving the signals transmitted by a transmitter made using either MOS or bipolar technology, for example, the ECL (Emitter-Coupled Logic) type, characterized by a slight deviation of the output signals which is typically 0.8 volts.
The invention aims to solve the first problem, and more particularly in the difficult context of the second problem caused by the dissipation of the characteristics and the compatibility of the manufacturing technologies, by using an impedance adaptation process.
It is common knowledge that adapting the impedance for electromagnetic wave transmission can be accomplished in series or in parallel. Series adaptation consists of adapting the transmitter's output impedance to the line's characteristic impedance, leaving open its terminal close to the receiver. This adaptation has the advantage of not requiring static consumption. However, it has the disadvantage of being very sensitive to impedance variations on the line and of carrying full-swing signals. The result is great dynamic consumption and a reduced bandwidth, despite the advantage of not having to amplify the reception signal. Finally, it has been seen that the transmitter's output impedance in particular is poorly defined during switching, so that the adaptation cannot be optimized and varies with utilization conditions.
Parallel adaptation consists of adapting the receiver's input impedance to the line's characteristic impedance. This has the advantage of applying to a wide band of frequencies, but it has the dual disadvantage of commonly involving static consumption and using a signal with reduced deviation at the line's output. Moreover, it is difficult to adapt the receiver's input impedance to the line's characteristic impedance because signal transmission between integrated circuits has the disadvantage of having a heterogenous structure. The coaxial cable of the transmission line is frequently connected on each side, using a coaxial connector, to a printed circuit equipped with the box in an integrated circuit into which the transmitter or the receiver is built. Thus, the impedance of the connection of the coaxial cable to the integrated circuit varies significantly and hence creates interference reactance. However, it is desirable to maintain the characteristic impedance over the longest possible length of each transmission line. One common solution to this problem consists of adapting the receiver's input impedance to the transmission line's characteristic impedance by connecting an adaptation resistor at the end of the line. This resistor must have a fixed and specific value, ordinarily 50 ohms. In MOS technology, the technological drifts between integrated circuits prevent such a resistor from being integrated. Thus, the adaptation resistor is placed external to the integrated circuit as close as possible to the circuit. However, practice has shown that, even without reactance, and if the characteristic impedance can be maintained over the entire length of the line, a substantial length of the line is not adapted if the adaptation resistor is placed before the entry to the integrated circuit at the high frequencies that are planned (on the order of several GHz). One other possibility consists of entering and exiting the integrated circuit to place the adaptation resistor at the end of the line, external to the integrated circuit. But this solution has the disadvantage of requiring two output terminals instead of one per line, plus the installation of the same number of resistors as lines on the integrated circuit. Furthermore, the interference reactance on each line would be doubled.
The solution that is the subject of this invention consists of a parallel impedance adaptation process capable of correctly and reliably overcoming all the constraints posed by the two aforementioned problems.